Detailed Description of the Invention
1. Technical Field of the Invention
The present invention relates to a charged roller type electrophotographic copying apparatus, and more particularly to its charged roller.
2. Prior Art
In an electrophotographic copying apparatus for copying/printing using toner conventionally used, the copying operation process is first started with an operation to charge a photoreceptor (generally drum-shaped), and in the case of performing the charging operation by atmospheric discharge, occurrence of ozone cannot be avoided, and this has posed a problem in terms of environmental pollution. As one of the solutions, there has been developed a charged roller type for charging by bringing a charged roller into contact with a photoreceptor to transfer the charge.
In the type for charging a photoreceptor using a charged roller, the photoreceptor is brought into contact with a cylindrical charged member, which has been charged, to thereby transfer the charge and charge the photoreceptor, and therefore, occurrence of ozone is restrained.
In a charged roller composed of this conventionally-used charging member structured into a cylindrical shape, there are two systems: film-shaped application system and brush-shaped application system as a typical application state of the charged member. When applied as the film-shaped system, semiconductive synthetic resin film is wound around/mounted to an aluminum tube (core body) through a conductive rubber layer when it is used (hereinafter referred to as film roll). On the other hand, when applied as the brush-shaped system, staple fiber of semiconductive synthetic fiber is uniformly electro-deposited on the outer periphery of the core body of an aluminum tube by the electrostatic flocking technique (so-called flocking method) (hereinafter, referred to as brush roll) when it is used.
In both the film roll system and brush roll system, however, no sufficiently satisfactory result was obtained because of the following problems:
That is, in the film roll system, it becomes difficult to maintain the uniformity in contact pressure between a photoreceptor and the film roll with an increase in the copying area (specially width), and as a result, charging spots tend to occur. Also, because of a mechanism in which rigid film rotationally contacts the photoreceptor structurally, the photoreceptor is easily worn out, and is easily damaged, and therefore, it has been designed to have such structure as to contact in a line contact state giving consideration to reducing the contact area as far as possible in order to avoid such a problem. However, this measure goes against it, the reduced contact width lowers the charging efficiency to cause a corona discharge-like charged state, and therefore, an effect of restraining the occurrence of ozone, which is the object, also deteriorates, and further, the risk of surface deterioration of the roll itself is also increased when higher voltage is applied. Therefore, design is compelled to be made by looking for common ground in the contact width, resulting in the difficulties that only an apparatus having insufficient functions and prone to cause image quality defects can be manufactured.
On the other hand, even in the brush roll system, in terms of selection of the optimum flexural hardness and size of fiber which forms the brush, and securing of an optimum, uniform filling length and uniformity of filling density, it is difficult to manufacture brushes in which all of the requirements meet the respective optimum states, and it is also difficult to look for the optimum charging conditions with an apparatus provided with such a charging device for setting and operating.
When the flexural hardness of filling fiber is too high or the rotating speed is too fast, the brush roll system suffered from problems in terms of service life of the apparatus and maintenance of uniform charging such as: the photoreceptor is easily damaged; falling-off of the filling fiber cannot be completely prevented; and it is difficult to secure exceedingly uniform filling density.
Further, in the contact at a cross-section of the foregoing fiber, the electric resistance of the fiber can be determined by the volume resistance value, but in the contact charging based on the volume resistance value, it is inferior in the stability of resistance. In other words, the resistance characteristics tend to vary by environmental changes or changes over time. This is presumed to be because in the case of conductive powder dispersed in the fiber, for example, carbon black, it might move inside rather than on the surface of the fiber to change the state of dispersion. In addition, image defects based on pinhole defects on the surface of the photoreceptor tend to occur including the foregoing two systems. Therefore, it is also necessary to strictly control so as to use a non-defective photoreceptor.
As new means for improving the two foregoing known charging means, the inventor et al of the present invention previously developed a method using polyamide elastomer for contact charging, filed an application for patent, and the method was disclosed in Japanese Patent Laid-Open Application No. 9-230673.
The technique disclosed in the invention is to select polyamide elastomer as material for charged roll, to disperse and knead a conductive agent, preferably carbon black to this material, to impart a volume resistance value of 10.sup.3 to 10.sup.12 .OMEGA..multidot.cm to the elastomer, to cut this sheet to a predetermined size and to bring the cross-section into contact with the surface of the photoreceptor for charging it.
Problems to be Solved by the Invention
An electrophotographic copying apparatus according to the invention disclosed in the foregoing Japanese Published Unexamined Patent Application is excellent compared with conventional, known charged roller type electrophotographic copying apparatuses, is capable of eliminating many defects of the latter, and commercialization has already been attempted. In the durability test during the commercialization test stage, however, points to be further improved in the durability aspect were found, and it turned out that it is indispensable to solve them.
That is, at a point of time whereat 5,000 sheets have been copied, although there are few problems in practical use, faint streak-shaped density spots start to be seen in the copied/printed image. Further, on the 10,000.sup.th sheet, toner adhered on the photoreceptor starts to be seen around the contact portion of the sheet although slightly, and the tip end portion of the sheet starts to enter a seemingly shaved state. This phenomenon is presumed to have appeared for the following reason: because of charging at the cross-section of a thin sheet in line contact, all charge are concentrated on the tip end whenever voltage is applied, whereby the dispersed state of carbon black at the tip end portion varies to cause fluctuation in the volume resistance value; and resin, which is the matrix, itself starts to deteriorate.